When imaging the retina (or fundus, these terms will be used interchangeably) of an eye, some of the illumination light beam from a light source may be reflected and/or scattered from the cornea, the iris and the crystalline lens of the eye. It is possible that this undesired reflected and/or scattered light resulting from the interaction of the illumination light beam with the anterior segment of the eye can be mixed with the imaging beam which comprises desired light reflected/scattered from the retina of the eye for photographic imaging or observational purposes. This undesirable mixing can result in the appearance of flair light, artifact, haze or ghost images on the retina image.
To get rid of these undesirable effects, an annular ring-shaped illumination light with a selected annular width and a numerical aperture can be focused at the cornea region to illuminate a large area of the retina, and the imaging path can be designed to occupy a space inside of the annular ring on the cornea. When the eye is properly positioned the illumination path has no overlap with the imaging path at the cornea, iris and crystalline lens so that light from the illumination beam will not be reflected and/or scattered into the imaging beam. However, to ensure that there is no undesirable effect on the retina image, a correct working distance between the objective lens of the fundus camera and the patient's eye must be maintained.
Typically, the maintenance of the working distance is accomplished by providing one (or more) light emitting element(s) (usually of the near infrared spectral range) behind a first lens at one side of the optical axis of an objective lens of the fundus camera and one (or more) corresponding light detecting element(s) behind a second lens at the other side of the optical axis. The light from the light emitting element is either focused or collimated by the first lens and directed to intersect the optical axis of the objective lens at a predetermined point and the light as reflected at the corneal surface is focused at the light sensing element by the second lens (see for example U.S. Pat. No. 4,436,389, U.S. Pat. No. 6,220,706). When the light sensing element receives a maximum signal, the working distance is considered correct.
A major problem associated with this approach is that the working distance determined by the approach is not always correct and is highly dependent on the surface profile and orientation of the cornea surface at the light intersection point. In other words, this approach will only work if the cornea surface that intersects with the light beam for working distance detection is not tilted with respect to the optical axis of the objective lens of the fundus camera. Unfortunately, this is not always the case. For example, when a doctor wants to image the peripheral region of the retina, he or she needs to orient the fundus camera at an angle with respect to the optical axis of the eye, and in such a case, the required correct working distance will in fact be slightly different from that for the central or non-peripheral retina imaging case. Meanwhile, the cornea region that intersects with the light beam for working distance detection will generally not be the apex with a normal that is coaxial with the axis of the objective lens. As a result, the distance thus determined by such an approach will be wrong. Similarly, if the cornea surface profile of the patient eye is not ideal, such as for those who have keratoconus or have had LASIK surgery, a wrong working distance will also be established using this prior art approach. The consequence is a retina image that will have undesirable flair or another artifact.